I have just come across a problem that some of you maybe interested in.

I read an RR awhile ago where a 990 sprung a bad oil leak which turned out to be the oil pressure sender unit. This is not really an uncommon occurrence in the automotive world. The guy was in a remote area and it took a week to get the part sorted.
So being someone who frequents remote areas I thought I should replace mine with a new one, and because they were so cheap (
I went to replace it today as I have the bike stripped down and I noticed the new copper washer would not go down flush to the mating surface of the sender. The corner where the thread meets the sealing surface was very untidy and oversize, I checked the spare and it was the same. The color of the unit is different to the original but it has the same ID stampings. So I stuck it in the lathe and carefully machined the thread diameter all the way to mating surface. Easy... job done... or so I thought.

I then noticed that the end of the thread finished well short of the mating surface and it struck me that I may need a thicker copper washer. I screwed the unit in without the washer and measured with feeler guages the clearance after the sender lightly bottomed out on the thread. The clearance was exactly 0,8 mm. I obviously then measured the new washer... 0.8 mm So of course I'm going to need a thicker washer.

The problem is I'm an inquisitive bastard and I wanted to know how this could be. I checked the part numbers to make sure I had the correct parts, all good. I then measured the old copper washer which was 0.78, so that was the same. I then screwed the old sender unit in and measured the clearance without the washer and strangely enough it also bottomed out at 0.8 mm ..... WTF. how had this been sealing so well for 60000km.
After much head scratching I looked at the copper sealing ring decription on the packet and it claimed to be 1.0 mm thick which if true would solve the problem.
I found a new copper washer in my collection which was 1.2 mm thick and I was good to go.

So the point of this long winded post is that if anyone like me has been carrying a new spare sender around with them, its probably not going to fit without some preparation.

When I measured the old unit at 0.8 mm I only lightly bottomed the thread with a socket, I guess I could force it another 1/8 of a turn which must be just enough to put a bit of pressure on the washer. Certainly not much room for error.

Most pressure switches I've dealt with were 1/8" National Pipe Thread.. which is a tapered thread seal...

Maybe you're onto why the leaks occur? This has a straight thread with a crush washer?

Interesting.

Yes your right, lots of senders are NPT but this is a parallel thread and uses a crush washer.
Ive only ever seen one documented failure but because a genuine replacement was so cheap I thought it was a smart thing to do. I know this probably means that the new one will probably let me down one day in the most inconvenient place.